Low Ionization Nuclear Emission-Line Regions (LINERs), which exist
 in the nuclei of a large fraction of galaxies, may be the weakest
 manifestation of the quasar/AGN phenomenon. Alternatively, the
 emission lines may be produced in shocked gas or HII regions
 associated with clusters of unusually-hot stars. Choosing between
 these alternatives is required in order to correctly place LINERs
 on (or remove them from) the AGN map. Since LINERs are so common,
 this will have far-reaching implications for the number of AGNs at
 the lowest luminosities, their evolution, and the X-ray background.

 We propose to use the ISO SWS to observe five LINER galaxies in
 which we have recently discovered bright compact ultraviolet
 nuclear sources in HST images, and two LINERs in which nuclear
 UV sources are absent.  All seven of our targets display similar
 optical line ratios independent of the presence or absence of the
 central UV source. By searching for IR differences among the two
 types of object,  our proposed ISO observations will determine
 whether different excitation mechanisms are operating in the
 UV-bright and UV-dark LINERs, or whether the UV-dark LINERs are
 obscured sources. The IR measurements will provide important and
 previously-unavailable diagnostics for determining the excitation
 mechanisms in each of the two types of LINERs. We will complement
 our ISO spectral measurements with UV and X-ray spectroscopy of
 our sources by HST and ASCA. The ISO observations will, for the
 first time, provide comprehensive multi-wavelength data for a
 sample of LINER galaxies and new and unique information about
 the nature of the ionizing sources in these objects.